Add Dockerfile pinning node:22-alpine by sha256

Non-server image: brings up the dev environment and runs make check, per
repo policy on Dockerfiles for non-server repos. Base image hash matches
the sneak/prompts template (node 22-alpine, 2026-02-22).

Dockerfile

@@ -0,0 +1,11 @@
+# node 22-alpine, 2026-02-22
+FROM node@sha256:e4bf2a82ad0a4037d28035ae71529873c069b13eb0455466ae0bc13363826e34
+
+RUN apk add --no-cache make
+
+WORKDIR /app
+COPY package.json yarn.lock ./
+RUN yarn install --frozen-lockfile
+COPY . .
+
+RUN make check